The present invention relates to an accelerometer for detecting acceleration by bending a beam, especially, relates to a silicon integrated accelerometer wherein a signal processing circuit with sensor elements in a mixed manner is formed on a structure by using micro machine technique, thereby detecting acceleration components in multi-axial directions.
In recent years, in place of accelerometers capable of detecting the acceleration in only one axis, multi-axial accelerometers capable of detecting the acceleration components in biaxial directions or in triaxial directions have been known (see Japanese Patent Application Laid-open No. 9-113534 (1997) etc.).
First of all, as a first example according to the prior art, a structure of an accelerometer for detecting acceleration components in triaxial directions will be described with reference to FIGS. 1 to 3.
FIG. 1 is a sectional view showing an accelerometer 102 for detecting acceleration components in triaxial directions. This accelerometer 102 has a hollow cylindrical support 104 and a circular silicon substrate 105 bonded on the support 104. A columnar seismic mass 107 consisting of the Pyrex glass is bonded to a center of a bottom surface of the silicon substrate 105.
In the silicon substrate 105, a diaphragm 108 formed into a thin thickness is structured between a peripheral portion 105a bonded to the support 104 and a central portion 105b bonded to the seismic mass 107.
FIG. 2 is a top view showing the accelerometer 102. As the center of the silicon substrate 102 is regarded as the origin, piezo-resistance elements Rx1, Rx2, Rx3, and Rx4 are formed on a top surface of the portion 108, and each two of them positioned in a positive or negative portion on the X axis, respectively.
Samely, piezo-resistance elements Ry1, Ry2, Ry3, and Ry4 are formed on the portion 108, each two of them positioned in a positive or negative portion on the Y axis, respectively. Further, piezo-resistance elements Rz1, Rz2, Rz3, and Rz4 are arranged at position closely adjacent to the piezo-resistance elements Rx1, Rx2, Rx3, and Rx4 in parallel to each of them on the X axis, respectively.
Each group of piezo-resistance elements Rx1-Rx4, piezo-resistance elements Ry1-Ry4, and piezo-resistance elements Rz1-Rz4 composes a bridge circuit, respectively. Before the measurement of the acceleration, each of these three bridge circuits maintains a null balance condition indicating zero output.
As described the above structure, when the acceleration is applied to the accelerometer 102, a stress is applied at the diaphragm 108 by weight of the seismic mass 107, so that the diaphragm 108 is deformed mechanically.
This mechanical deformation causes the change in the resistance of the piezo-resistance elements disposed in the direction to which the stress is applied on the flexible part 108, and the bridge circuit is unbalanced to generate an electric output. Because the resistance of the piezo-resistance elements composing the bridge circuits are changed according to the magnitude and direction of the acceleration, the acceleration components in the three axis (namely, the X, Y, and Z axis) directions can be measured by measuring the changes in the resistance of respective bridge circuits.
FIG. 3 shows an example of a bridge circuit composed of the piezo-resistance elements Rz1-Rz4 for detecting the acceleration component in the Z axis (hereinafter referred to as xe2x80x9cZ axis acceleration componentxe2x80x9d). In FIG. 1, when the acceleration is applied to an upward direction, the piezo-resistance elements Rz1 and Rz4 are applied a negative (minus) stress and the Rz2 and Rz3 are applied a positive (plus) stress. Hence, the bridge circuits induce imbalance, and a potential difference Vab between bridge terminals occurs, thus detecting the Z axis acceleration component.
Furthermore, as a second example according to the prior art, a current detection-type accelerometer having a differential amplifier circuit therein will be described with reference to FIGS. 4A and 4b (see Japanese Patent Application Laid-open No. 6-207948 (1994)).
In FIG. 4A, diffusion layers 151 and 152 having electroconductivity opposite to that of a semiconductor substrate 150 are formed on the top surface of the semiconductor substrate 150. Moreover, an electrode 153 is provided above the semiconductor substrate 150 with a given separation, being between the diffusion layers 151 and 152. This electrode 153 is defined as a movable electrode of beam structure. In this way, a MIS (Metal Insulator Semiconductor) transistor is structured with the use of air as an insulator film.
Then, by the application of an appropriate voltage on the electrode 153, the inversion layer 154 is formed just below the electrode 153, and the inversion layer 154 conducts electric current between the diffusion layer 151 and the diffusion layer 152, thus flowing a current proportional to a capacitance between the electrode 153 and the semiconductor substrate 150.
Then, if the acceleration is exerted in the X direction perpendicular to the semiconductor substrate 150, the electrode 153 deforms in a direction perpendicular to the substrate surface and the distance between the electrode 153 and the substrate surface changes. As a result, the capacitance changes, and then the amount of current flowing in the inversion layer 154 change, thereby measuring the acceleration proportional to the change in the amount of current.
Further, in FIG. 4B, when a differential amplifier circuit is structured by using two MOS transistors, the acceleration in the X axis effects both of the two MOS transistors 160, 161 equally, so that the amount of currents flowing in the MOS transistors 160, 161 do not have a difference. However, with respect to the displacement of the electrodes in the Z direction, an overlapping width Wa of the MOS transistor 160 and an overlapping width Wb in the MOS transistor 161 are in such a relationship that when the one increases, the other decreases. As a result of this relationship, the currents flowing in the two MOS transistors 160, 161 also vary according to respective overlapping widths Wa, Wb, thus detecting only the acceleration in the Z direction.
In various systems having pressure sensors or accelerometers, miniaturizing the sensors and making these power consumption lower have been in progress. Recently, there is a sensor module in which a peripheral circuit as well as sensor bodies are integrated on a sensor substrate.
In addition, in consideration of the connection of this kind of modules to computers, a sensor equipped with an A/D (analog to digital) conversion function that directly gives a digital output has been developed. As an A/D converter circuit for integrating this kind of sensor, there is a simplified A/D converter in which a plurality of CMOS inverters each of which has a different logic threshold are parallel-connected.
In the first example according to the prior art, as shown in FIG. 1 above, semiconductor diffusion resistance layers as piezo-resistance elements are formed on the flexible part 108 and a bridge circuit is structured by the four piezo-resistance elements, as a unit, thus measuring the acceleration by means of the piezo-resistance effect.
However, in the case of a structure with the use of the semiconductor diffusion resistance layers, there is no function for adjusting the fabrication imbalance in the voltage of an operating point of the bridge circuit and hence offset voltage cannot be reduced sufficiently. Therefore, it is necessary to provide separately a compensation circuit for compensating an offset voltage due to the fabrication imbalance, thus increasing a fabrication cost.
Furthermore, in a configuration with such a bridge circuit, an amplifying function for amplifying a detected signal cannot be provided and hence the output signal level is low. Especially in accelerometers, since the output signal level is low and detection sensitivity is insufficient, a load of an amplifier circuit for amplifying that small output signal becomes heavy, and larger power consumption is required.
Furthermore, when the acceleration is measured by using such a bridge circuit, an adjustment function for adjusting the detection sensitivities according to detected components of the off-axis except a sensitive axis to which the stress is applied is lacking and therefore the accuracy of the measurement is questionable.
On the other hand, in the case of the second example according to the prior art shown in FIG. 4, a capacitance-type structure is employed wherein just beneath the electrode 153 is the air as an insulator film and the acceleration is measured by detecting the current proportional to the change in the capacitance.
However, such a capacitance-type accelerometer has not been made up with the use of piezo-resistance elements and cannot be structured by employing a known CMOS LSI fabrication technology, as it is.
Consequently, there is a problem that its fabrication process becomes complicated and hence an inexpensive sensor cannot be fabricated under maintaining a low-cost for fabrication.
It is also impossible for the offset voltage of a differential circuit to be compensated because the gate electrodes of the two input transistors are common.
In addition, the accelerometer for detecting acceleration components in the triaxial directions has a problem that the compensation of the offset voltages and the improvement of all the S/N ratios according to the triaxial directions cannot be performed at the same time.
An object of the present invention is to provide a silicon integrated accelerometer that has an inexpensive sensor body and has a simplified circuit configuration and is capable of removing the effects of the offset outputs regardless of imbalance in fabrication conditions.
Further, another object of the present invention is to provide a silicon integrated accelerometer capable of improving the measurement accuracy by suppressing sensitivities to off-axis as well as by improving the detection sensitivity to the sensitive axis.
The present invention is a silicon integrated accelerometer comprising a fixed support base, a movable seismic mass, and beams having a thin thickness connecting the support base and the seismic mass, capable of measuring acceleration by using bending of the beams based on a stress, the sensor comprising:
a plurality of strain detecting elements, arranged at stress concentration portions of the beams, for detecting the stress applied in the beams; and
differential amplifier circuits, arranged in the support base, for detecting the output values outputted by a plurality of strain detecting elements, wherein
the output value according to the sensitive axis acceleration component in a detection axis direction, to which the stress is applied, is detected as a differential mode and the output value according to the other axis components in the other axis directions is detected as a common mode.
In this configuration, a plurality of strain detecting elements are MOS (metal oxide semiconductor) transistors.
The stress concentration portion of the beams are a boundary portion on the beams connected to at least one of the support base and the seismic mass.
It is possible that the support base and the seismic mass, and the beams are structured by using (100)-oriented silicon single crystal substrate and the beams are arranged in a direction parallel to the crystallographic axis  less than 011 greater than .
The sensor may be structured by a configuration wherein the seismic mass surrounds the support base.
The sensor may be structured by a configuration wherein at least one of beams are arranged at each of the both sides of either the support base or the seismic mass, and the differential amplifier circuits is structured by arranging two strain detecting elements at the boundary portions between each the beam and the support base.
The sensor may be structured by a configuration at least one of beams are arranged at each of the both sides of either the support base or the seismic mass, and the differential amplifier circuits is structured by arranging two strain detecting elements at the boundary portions between each the beam and the seismic mass.
The sensor may be structured by a configuration wherein at least one of beams are arranged at each of the both sides of either the support base or the seismic mass, and the differential amplifier circuits is structured by arranging one strain detecting element at the boundary portion between each the beam and the support base and by arranging one strain detecting element at the boundary portion between each the beam and the seismic mass.
The sensor may be further comprised of two current sources for driving two pairs of the differential amplifier circuits detecting an acceleration component in a uni-axial direction, and a switch for switching of the two current sources at high frequency.